Hi there, I have a Samsung S3 NZ new on the telecom network with a telecom sim card. I arrived in colorado 5 days ago and my phone connected to AT&T and was able to make calls. We moved locations by 2 hours drive to where AT&T is available still but I get the above message.

your sim doesn't allow connection to this network

There are other kiwis here who are telecom customers using their phones on the AT&T network...we have a strong signal here. My phone shows a strong signal too but i get the message.